"That's who he'll be standing beside when he announces," Tester said on ABC's "TopLine" webcast, referencing the dinner on Saturday where Rehberg will formally announce his candidacy and where Bachmann will be a featured speaker.

"She has just put out a proposal to make draconian cuts to the VA," Tester said, wondering aloud whether Rehberg would endorse the Minnesotan's proposals, which are part of a larger, $400 billion spending-cut proposal.

Bachmann's sought to build a national profile with a series of speeches and a trip to Iowa to flirt with her own presidential bid in 2012. She's considered a popular figure in the Tea Party movement, but she's drawn more national scrutiny — for instance, in a "Saturday Night Live" speech lampooning her Tea Party response to the State of the Union last week. Tester's comment suggests that Democrats believe Bachmann could be a drag on certain candidates in the coming cycle.

Tester is one of the 23 Democrats up for reelection in 2012, and he's expected to face one of the most difficult challenges in his native Montana, with its right-leaning electorate.

Recruiting Rehberg, who's been elected five times statewide as Montana's lone representative, was seen as a coup for national Republicans, who are hoping to reclaim the seat for the GOP.

"It's going to be an interesting campaign," Tester said. "He's got a record, too, and we'll have the opportunity to compare those records."